The present invention generally relates to optical recording devices, and, more particularly, to an optical recording device that can set an optimum focus offset value and a method of determining an optimum focus offset value.
To record information on a recording face of an optical recording medium and then reproduce the information from the optical recording medium, it is necessary to perform a control operation to focus the light emitted from a light source, such as a laser diode, onto the recording face of the optical recording medium, and to maintain the “just-focus” state. This control operation is referred to as a “focus servo control operation”.
FIG. 1 shows an example of a conventional optical recording device 1 that records information on an optical recording medium 10 of a magneto-optical disk type, or reads out information from the optical recording medium 10. The optical recording device 1 shown in FIG. 1 includes a control unit 2 and a disk enclosure 3.
The control unit 2 includes a host interface 11, a buffer memory 12, an MPU 13, an optical disk controller 14, a read/write LSI 15, a DSP 16, a focusing error signal detecting circuit 17, a tracking error signal detecting circuit 18, a tracking zero-cross detecting circuit 19, and drivers 20 through 23. The enclosure 3 includes a laser diode unit 31, an ID/MO signal detector 32, a head amplifier 33, a spindle motor 34, a magnetic field applier 35, a focusing error detector 36a, a tracking error detector 36b, a focusing actuator 37, a tracking actuator 38, and an object lens system 40.
The host interface 11 interfaces with a host device such as a personal computer. The data transmitted between the host interface 11 and the host device are temporarily stored in the buffer memory 12. The MPU 13 and the disk controller 14 control the operations of the optical recording device 1. memory 12. The MPU 13 and the disk controller 14 control the operations of the optical recording device 1.
The read/write LSI 15 modulates and demodulates data. To write data on the optical recording medium 10, the read/write LSI 15 modulates the write data and supplies the modulated write data to the laser diode unit 31. On the other hand, to read data from the optical recording medium 10, the read/write LSI 15 controls the laser diode unit 13 to emit read light.
The read light emitted from the laser diode unit 31 onto the optical recording medium 10 is reflected by the optical recording medium 10, and thus is supplied to the ID/MO signal detector 32, the focusing error detector 36a, and the tracking error detector 36b. The Id./MO signal detector 32 detects the ID/MO signal component from the read light reflected by the optical recording medium 10, and sends the detected ID/MO signal to the head amplifier 33. The head amplifier 33 in turn amplifies the ID/MO signal, and sends the amplified ID/MO signal to the read/write LSI 15. The read/write LSI 15 demodulates ID data from the ID/MO signal. The data demodulated by the read/write LSI 15 is stored in the buffer memory 12.
The focusing error detector 36a converts the incident light to an electric signal, and sends the electric signal to the focusing error signal detecting circuit 17. Based on the electric signal sent from the focusing error detector 36a, the focusing error signal detecting circuit 17 generates a focusing error signal E1.
The focusing error signal E1 generated by the focusing error signal detecting circuit 17 is then supplied to the DSP 16. Based on the focusing error signal E1, the DSP 16 generates a focus control signal, and then sends the focus control signal to the driver 22. Based on the focus control signal supplied from the DSP 16, the driver 22 supplies a drive current to the focusing actuator 37. Based on the drive current supplied from the driver 22, the focusing actuator 37 moves the object lens system 40 in the focusing direction. By doing so, the laser light emitted from the laser diode unit 31 is focused on the optical recording medium 10.
The tracking error detector 36b converts the incident light to an electric signal, and sends the electric signal to the tracking error signal detecting circuit 18. Based on the electric signal supplied from the tracking error detector 36b, the tracking error signal detecting circuit 18 generates a tracking error signal E2. The tracking error signal E2 detected by the tracking error signal detecting circuit 18 is then supplied to the DSP 16 and the tracking zero-cross signal detecting circuit 19. Based on the tracking error signal E2, the tracking zero-cross signal detecting circuit 19 generates a tracking zero-cross signal E3, and supplies the tracking zero-cross signal E3 to the DSP 16. Based on the tracking error signal E2 and the tracking zero-cross signal E3, the DSP 16 generates a tracking control signal, and supplies the tracking control signal to the driver 23.
Based on the tracking control signal supplied from the DSP 16, the driver 23 supplies a drive current to the tracking actuator 38. The tracking actuator 38 is driven based on the drive current supplied from the driver 23, and moves the object lens system 40 in the radial direction of the optical recording medium 10, thereby performing a tracking operation.
The MPU 13 generates a spindle motor control signal, and supplies the spindle motor control signal to the driver 20. Based on the spindle motor control signal supplied from the MPU 13, the driver 20 rotates the spindle motor 34.
The MPU 13 also generates a magnetic field control signal, and supplies the magnetic field control signal to the driver 21. Based on the magnetic field control signal supplied from the MPU 13, the driver 21 supplies a drive current to the magnetic field applier 35. The magnetic field applier 35 then generates a bias magnetic field in accordance with the drive current supplied from the driver 21. The bias magnetic field generated by the magnetic field applier 35 is applied to the optical recording medium 10 for information recording and/or reproduction.
The optical recording device 1 is designed in such a manner that the optical recording device 1 is in a “just focus” state when a focus servo control operation is performed. However, a shift from the “just focus” position occurs over a time interval, and therefore it is necessary to correct such a shift so that the optical recording device 1 can return to the “just focus” state. To do so, it is necessary to determine the location of the “just focus” position.
FIG. 2 shows the relationship between the recording face of a recording medium and a laser beam emitted onto the recording face. In FIG. 2, reference numeral 203 indicates a laser beam focused onto the recording face 202 of the optical recording medium 10. Reference numeral 204 indicates a laser beam emitted onto the recording face 202 of the optical recording medium 10 in a case where a negative offset value is given, and reference numeral 205 indicates a laser beam reflected from the recording face 202 in such a case. Reference numeral 206 indicates a laser beam emitted onto the recording face 202 of the optical recording medium 10 in a case where a positive offset value is given to the focus error signal, and reference numeral 207 indicates a laser beam reflected from the recording face 202 in such a case. As shown in FIG. 2, under the focus servo control, the “just focus” point can be moved forward or backward on the recording face of the optical disk by giving an offset value to the focus error signal.
Examples of conventional methods of searching for a “just focus” point by adjusting a focus offset value includes a method in which a focus offset value is changed while a focus servo control operation and a tracking servo control operation are performed, and the focus offset value, at which the amplitude of the ID signal becomes greatest and the “just focus” point is located, is determined to be the optimum focus offset value.
Alternatively, the focus offset value may be changed while a focus servo control operation and a tracking servo control operation are both performed, and the point where the amplitude of the MO signal becomes greatest is determined to be the “just focus” point. Further, the point where the maximum quantity of light reflected from the optical recording medium is determined to be the “just focus” point. In yet another example of the conventional method, a tracking servo control is stopped, and the point where the amplitude of the tracking error signal becomes greatest is determined to be the “just focus” point.
In a conventional optical recording device, the recording density of an optical recording medium is relatively low. This is why the read error rate can be constantly maintained below a predetermined value by determining the “just focus” point and the optimum focus offset value in the above methods.
Continuing the trend of recent years, however, the recording density of recording media is becoming higher, and the tolerable range for defocusing with a read error rate below a predetermined value is becoming narrower. With a high-density recording medium, even if the focus offset value at which the “just focus” point is located is determined to be the optimum focus offset value in any of the above conventional methods, the read error data cannot be maintained below a predetermined value.
To solve this problem, it is necessary to determine a more accurate optimum quantity of light for recording and reproducing, and a more accurate optimum focus offset value.
Japanese Laid-Open Patent Application No. 8-129754 discloses a method of determining an optimum quantity of recording light. In this method, the optimum quantity of recording light is set so that the deviation signal of two recording patterns becomes zero with the quantity of recording light. However, this method requires a circuit prepared especially for measuring the deviation signal, which results in higher costs.
Japanese Laid-Open Patent Application No. 2001-23165 discloses a method of determining an optimum focus offset value. In this method, the focus offset value is changed while the quantity of recording light is maintained at a constant value, and information is recorded on an optical recording medium with the varying focus offset value. The tolerable range of the focus offset value for reproducing information is determined from whether the information recorded on the optical recording medium can be reproduced. The quantity of recording light is then decreased while the focus offset value is varied, and information is recorded on the optical recording medium. The information recorded on the optical recording medium is then reproduced, and the tolerable range of the focus offset value for reproducing the information is determined from the reproduced results. These procedures are repeated, and the optimum focus offset value is determined to be the focus offset value at which the quantity of recording light becomes smallest. The optimum quantity of recording light is then determined by adding a predetermined value to the smallest quantity of recording light. In this conventional method, however, a true optimum quantity of recording light cannot be determined. Also in this conventional method, an optimum focus offset value cannot be determined in a case where the curve representing the read error rate to the quantity of light for recording and reproducing or the curve representing the read error rate to the focus offset value is not symmetrical with respect to the point of the lowest error rate.
Japanese Laid-Open Patent Application No. 2001-23165 discloses another method of determining an optimum focus offset value. In this method, the optimum focus offset value is determined to be the focus offset value at which a reproduction signal jitter becomes smallest. However, this method requires a circuit prepared especially for measuring the reproduction signal jitter, which results in higher costs.